The objective of this study is to assess tracking of bone mass during childhood and adolescence. Specifically, we will estimate the degree of tracking over 1 and 2 year intervals in girls between 3 and 17 years of age. Annual measurements of bone mineral content and density will be measured by dual energy x-ray absorptiometry for three years. A total of 245 girls have been enrolled in the study, and 167 have completed their second annual measurement. We are also evaluating the comparability of bone density measurements obtained by peripheral Quantitated Computed Tomography (pQCT) to those obtained by DEXA. This will allow us to determine which technology to use in future studies of bone mineralization in children.